As shown and described in my U.S. Pat. No. 3,220,234 of Nov. 30, 1965, the forming of pipe from a thin strip of ductile sheet metal by corrugating it and then helically coiling or winding the corrugated strip and forming a lock seam, is old.
The thinner the metal the greater the economy of manufacture and the ease of manufacture, therefore, structural improvements, particularly in the seam, are essential to compensate for the loss in the rigidity of the material. In use, the pipe may be subjected to relative high pressure, and while the corrugations facilitate the making of bends in its length, the seam, which is parallel with the corrugations, is placed under a strain sufficient, at times, to cause leakage at the seam. Also, in view of the light gauge of the metal and the narrowness of the strip forming the pipe, it is highly desirable that the seam be of a structure that will contribute to the rigidity of the pipe.